The present invention relates to a double energy radiography method, together with an appropriate calibration device for this method.
Double energy radiography consists of exposing an subject or a patient to radiation with two different energies, for which the attenuation properties of the materials constituting the subject or the patient do not vary in the same way. Using a model of their attenuation functions, one thus obtains information about the density and nature of the material crossed through. In particular, in the case of bone densitometry, the bone mass crossed can be calculated by distinguishing it from the contribution of soft tissues to attenuation of the radiation.
The theoretical basis of the method will be resumed briefly below.
The flux xc3x8, after crossing a length l of a material with linear attenuation coefficient xcexc from an initial radiation flux xc3x80, is equal to xc3x8=xc3x80exe2x88x92xcexc1. The attenuation measurement is named m, equal to   ln  ⁡      (                  I        0            I        )  
where I0 and I are signals measured by a same detector under the fluxes xc3x80 and xc3x8. In the case of a complex subject composed of a large number of materials (of index i), each one contributes to attenuation according to its length Li crossed by the rays.
However, each material can be expressed, for its attenuation property, as a linear combination of two base materials, according to the formula:
xe2x80x83xcexc=k1xcexc1+k2xcexc2,
where k1 and k2 are constant coefficients, and xcexc1 and xcexc2 represent the attenuation of these base materials, and the equivalent lengths A1 and A2 of the base materials crossed by the radiation by:   "AutoLeftMatch"      {                                                      A              1                        =                                          ∑                1                                  xe2x80x83                                            ⁢                              xe2x80x83                            ⁢                                                L                  i                                ·                                  k                  1                                                                                                                    A              2                        =                                          ∑                1                                  xe2x80x83                                            ⁢                              xe2x80x83                            ⁢                                                L                  i                                ·                                  k                  2                                                                        
By means of these equivalent lengths, the base materials can represent a subject even if in reality its composition is much more complex. Even if the materials composing the subject are different from the base materials, the breakdown has a meaning. In the case of examining living beings, the classic base materials are Plexiglas (polymethacrylate) to simulate soft tissues and hydroxyapatite to simulate bone tissues.
The system of equations linking the measurements to the attenuation xcexc1 and to the equivalent lengths Al of the base materials is linear and therefore simple to solve if the radiation is monochromatic. But this is not so for real situations, and the equivalent lengths are then given by more complicated mathematical models, such as:   "AutoLeftMatch"      {                                                      A              1                        =                                          a                0                            +                                                a                  1                                ·                mBE                            +                                                a                  2                                ·                mHE                            +                                                a                  3                                ·                mBE                ·                mHE                            +                                                a                  4                                ·                                  m                  BE                  2                                            +                                                a                  5                                ·                                  m                  HE                  2                                                                                                                    A              2                        =                                          b                0                            +                                                b                  1                                ·                mBE                            +                                                b                  2                                ·                mHE                            +                                                b                  3                                ·                mBE                ·                mHE                            +                                                b                  4                                ·                                  m                  BE                  2                                            +                                                b                  5                                ·                                  m                  HE                  2                                                                        
which generally provide sufficient precision, and where mBE and mHE represent the attenuation measurements at high and low energy, and xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d are coefficients which must be calculated beforehand by calibration.
This calibration requires a device which is often called a xe2x80x9cphantomxe2x80x9d and which is composed of base materials as described above and chosen in order to simulate the subject to be measured as closely as possible. These materials are distributed in the phantom in such a way as to provide regions where the lengths of materials, crossed by the radiation, are distributed differently.
A classic phantom is built up in the form of a staircase for each of the measuring materials, and the stairs are superposed in such a way that their steps are perpendicular. By making vertical rays cross the steps, one thus obtains all the required combinations between the diverse thicknesses of the two materials.
Another phantom is proposed in the U.S. Pat. No. 5,493,601 and comprises a series of tubes converging towards the source of the beam and provided with heights divided unequally between the two materials. The aim is to provide more exact measurements than previously, in particular because of the convergence of the tubes towards the source, which makes the length the rays pass through in the tubes coincide perfectly with the total height of the latter, and also reduces the scattered radiation by containment in the tubes. The measurements are thus a direct expression of the attenuation of the radiation through the phantom and make it possible to calculate the coefficients required reliably.
However, the same cannot be said for the measurements produced through the subject to undergo radiography, for which the favourable conditions described above cannot be assembled and the scattered radiation must not be neglected; it is even especially important for sources with a conical beam, often more so than for the primary radiation, which has followed a straight line trajectory from the source to the detector and is the only one useful for the measurement. The estimation and then correction of this scattered radiation requires specific processing. Thus, amongst other methods, it is possible to measure it separately and then to subtract it from the total radiation received by the detector. In order to do this, one uses a network of absorbent elements, such as lead balls arranged in such a way as to form a grid. The balls are glued on Plexiglas and set in a homogeneous pattern, regular, for example, for the lines and columns so as to allow interpolation between the balls, between the radiographed subject and the detector""s network of pixels. The rays intended to pass through these balls are intercepted totally, so that only the scattered radiation reaches the pixels of the detector located in the path of these rays. Digital interpolations then make it possible to estimate, with sufficient precision, the distribution of the scattered radiation over the whole of the detector""s pixels. This convenient method, however, has the disadvantage of having to submit the subject to a second irradiation, which may be difficult for living beings to accept.
On the other hand, one can also prevent the scattered radiation from reaching the detectors by providing them with a strict collimation for interception. The methods using this technique necessitate the use of source scanning which implies a long acquisition time and the risk that the subject may move during measurement.
Digital methods of various types have also been proposed to correct the influence of scattered radiation, but generally they are only suitable for particular subjects. It has to be concluded that the patent quoted above does not appear to propose any notable progress, since the scattered radiation is only separated from the measurements at the calibration stage.
The aim of the present invention is to correct the influence of the scattered radiation in another way, by allowing it to appear during calibration, but in an analogous manner to its behaviour during radiography of the subject, which makes it possible to correct it by an identical digital method, without fear of discordance of quality for this method.
To resume, the invention concerns above all a calibration device for a double energy radiography system, comprising a range of blocks with different thicknesses of a first material, characterised in that the blocks are provided with recesses and in that furthermore the device comprises inserts to fill up the recesses and including different distributions of heights, the heights and the thicknesses being considered in an identical direction, between the first material and a second material; together with a radiography method with a double energy conical beam, comprising a thickness estimation of materials of a radiographic subject by a digital combination of measurements of attenuation of energies, involving a coefficient calibration of the combination, characterised in that the calibration is measured by a radiography of a calibration device conforming to any one of the preceding claims, and that scattered radiation affecting the radiography of the calibration device is estimated while providing an estimation criterion used afterwards to estimate a scattered radiation affecting the radiography of the subject.